Some embodiments described herein relate generally to methods and apparatus for generating a temperature insensitive bandgap voltage reference using an input (supply) voltage that is lower than the base-emitter voltage (VBE) of a bipolar junction transistor (BJT).
Portable electronic/electrical systems that operate from a battery and/or from power harvested from the internal local environment typically consume small amounts of energy to prolong the system lifetime for a given amount of available energy. The energy budget for a portable system affects a widening set of applications due to a combination of requirements for smaller size (less battery volume, and hence less energy available), longer lifetimes (energy has to last longer), and/or more functionality (increased number of applications to implement with the same amount of energy). Many sensing applications use integrated circuits (ICs) or systems on chip (SoCs) to perform the sensing, computation, and communication functions that are used by a variety of applications.
In many cases, the time between sensor measurements can be relatively long such that the IC or SoC spends a substantial fraction of its lifetime in a standby mode. Known techniques reduce power consumed by the IC or SoC during standby mode, for example, by power gating unused circuit blocks. A subset of circuit blocks remains powered up during all times of device operation including, for example, a DC-DC regulator remains powered up to supply a stable operating voltage, VDD, which in turn involves a voltage reference to set the correct value for VDD. Typically, the most commonly used voltage reference is a bandgap reference that uses the silicon bandgap voltage to generate a temperature independent voltage reference.
An ideal voltage reference is independent of variation of power supply or temperature. A voltage reference is often included in many circuits, such as analog-to-digital converters, DC-DC converters, energy harvesting circuits, timing generation circuits, or other voltage regulators. Known implementations of bandgap reference typically involve the use of bipolar junction transistors (BJT) and large resistors to provide generate the bandgap voltage reference. Known conventional bandgap reference circuits, however, are limited to using input voltages higher than the base-emitter voltage (VBE) of a BJT because they inject a current into the BJT using a current source, current mirror, resistor, or switched capacitor network at a voltage higher than VBE.
Accordingly, for severely energy constrained electronic/electrical systems, a need exists for bandgap reference circuits with a low input voltage to allow for compatibility with energy harvesting and sub-threshold digital logic voltage levels. Additionally, a need exists to minimize power consumption for the bandgap reference circuit.